Recovery of CAP threshold and amplitude in chickens following kanamycin ototoxicity

Chickens were given a dose of kanamycin (400 mg/kg/d x 10 d) which destroyed hair cells over the basal 37-58% of the basilar papilla. Afterwards, the threshold and amplitude of the compound action potential were measured at recovery times ranging from 2 days to 10-20 weeks post-kanamycin treatment. At 2 days post-treatment, the thresholds at 1000, 2000 and 4000 Hz were elevated 40-60 dB while the thresholds at 250 and 500 Hz were elevated only 25 dB. By 10-20 weeks post-treatment, the threshold at 250 and 500 Hz had completely recovered whereas a residual threshold shift of 5 dB to 25 dB was present between 1000 to 4000 Hz. The maximum amplitude of the compound action potential was also reduced by more than 60% at all frequencies at 2 days post-treatment; however by 10-20 weeks post-treatment, the amplitude of the compound action potential had completely recovered at 500, 1000 and 2000 Hz. By contrast, the amplitude of the compound action potential at 4000 Hz was still reduced by more than 50% of its normal value 10-20 weeks post-treatment. The results of the present study indicate that the time course of recovery of the compound action potential is extremely slow and may lag behind the regeneration of hair cells by many weeks. The permanent deficits observed at the high frequencies could conceivably be due to functional deficits in regenerated hair cells, their afferent synapses or the loss of cochlear ganglion cells.